The present invention relates to a load detecting device for generators, in particular, relates to a load detecting device for detecting a change in load conditions of a generator driven by engines such as a diesel engine.
For a generator driven by such as a diesel engine, it is desired to make the permanent speed change and the transient speed difference small so that the output frequency or the revolutions of the generator is kept at a predetermined value even when an electrical load thereof varies. In a known control apparatus for controlling the revolutions of a generator, the permanent speed change and the transient speed difference are controlled to make them small by the following method, for instance. First, a signal representative of a difference between the actual revolutions and the set revolutions of a generator is integrated, and, by this integrated signal, the amount of supply fuel to the engine is controlled. Thus, the engine will be controlled so that the permanent speed change of the generator becomes zero, i.e., the speeds before and after the load change become equal. Second, a load current of the generator is detected, and the amount of supply fuel to the engine is controlled on the base of a change of the load current, before a substantial change of engine speed which might be caused by the load change appears. Thus, the engine will be controlled so that the transient speed difference of the generator becomes small, i.e., the engine speed is controlled to the set speed rapidly. Therefore, the known control apparatus reduces the permanent speed change to zero and reduces the transient speed difference small.
A conventional load detecting device used with the known speed control apparatus for detecting an electrical load of the generator includes a current transformer which detects a load current of the generator, a resistor connected in parallel with the transformer winding and providing a AC voltage signal corresponding with the detected load current, a rectifier rectifying the AC voltage signal, and a ripple filter smoothing the output of the rectifier and providing a DC voltage signal in proportion to the load current. An example of the load detecting apparatus has been shown in the Japanese Utility Model Laid Open Publication No. 7072/1983.
Such a prior load detecting device, however, has a problem that detection of a load change becomes slow due to a time constant of the ripple filter. This problem will become more clear by reference to FIG. 1.
FIG. 1 illustrates waveforms for explaining the operation of a prior load detecting device. FIG. 1(a) represents a load current detected by the current transformer, and FIG. 1(b) represents a voltage signal A and a DC voltage signal B. The voltage signal A is produced through a full-wave rectifier of an AC voltage representative of the load current. The DC voltage signal B is produced by smoothing the voltage signal A through the ripple filter. As apparent from FIG. 1, when a load change occurs, the level of DC voltage signal B changes gently due to the time constant of the ripple filter, so that the DC voltage signal B cannot follow an abrupt change of load. Therefore, the detection of the load change is delayed.